def prost(n):
    c = True
    for d in range(2, int(n ** 0.5)+1):
        if n % d == 0:
            c = False
            break
    return c

n = int(input())
if len(str(n)) != 4:
    print(0)
    exit()
numbs = []
ans = []

n1 = n // 1000
n2 = n // 100 % 10
n3 = n // 10 % 10
n4 = n % 10

numbs = [n1, n2, n3, n4, n1 * 10 + n2, n2 * 10 + n3, n3 * 10 + n4, n1 * 100 + n2 * 10 + n3, n2 * 100 + n3 * 10 + n4, n]

for x in numbs:
    if prost(x):
        ans.append(x)
    if ans:
        print(max(ans))
else:
    print(0)